Waste recovery process



Oct- 16, 1962 J. H. H1NKLE,.JR., ETAI. 3,058,803

WASTE RECOVERY PROCESS Original Filed Jan. 6. 1958 3,058,803 WASTERC/ERY PRGCESS John H. Hinkle, lr., Houston, and Everett 3. Weaver andLynnvai M. Smith, Pasadena, Tex., assigner-s, by mesne assignments, toHooker Chemical Corporation, New

York, N55., a corporation of New York Continuation of application Ser.No. 707,166, lan. 6, 1958.

This application Mar. 15, 1960, Ser. No. 15,118 6 Claims. (Cl. 23--88)This invention pertains to the recovery of components from gasiformmixtures, and, more particularly, the invention pertains to the recoveryof sulphur dioxide, sulphur trioxide, and uorine from lwaste gaseseffluent from chemical plants such as those for acidulation and/ orcalcination of phosphate minerals, e.g. phosphate rock.

This `application is a continuation of our copendng application S.N.707,166, led January 6, 1958, entitled Waste Recovery Process andApparatus, which applica- 'tion has now been abandoned.

A principal object of the invention is to provide a process forrecovering gasiform sulphur compounds and norine from gasiform mixtures.

Another object of the invention is to provide such process suitable foruse in recovering such gasiform materials from waste gases ofinstallations for acid, or other, treatment and/or calcination ofphosphate minerals.

A `further object of the invention is to provide such process which isefficient, is of minimum cost, is `free from troublesome operatingdiiculties, and which emit no noxious or other objectionable efuents.

A second principal object of the invention is to pro-v vide a process ofthe type described wherein the recovered materials are in valuableforms.

Another object of the invention is to provide such process with which:sulphur compounds are recovered in the form of high-grade clean orfresh sulphuric acid, even though the reaction from which the stockgases are derived is carried out with sulphuric acids of alkylation orother dirty grade. Thus, the recovered sulphuric acid is of much greatervalue than the sulphuric acid originally used in the process.

Another object of the invention is to provide such process from whichthere flows no liquid waste to contaminate sewage or other streams inthe plant area.

Another object of the invention is to provide a process vor alleviatingor eliminating air pollution problems and waste.

Industrial plants for the treatment of phosphatic minerals, and thevarious processes therein employed, are notable in -the noxiousness oftheir effluents, particularly their gasiform eiiluents. The stacks ofsuch plants, practically without exception, emit noxious materials inrelatively tremendous quantity, and are, therefore, a nuisance and oftena menace to surrounding areas, both Within and without the plant site.Furthermore, the emissions constitute a waste of potentially valuablematerials. In such plants employing sulphuric acid as a treatingmaterial, gaseous forms of sulphur are passed out of the stack.`Fluorine compounds of diverse constitution are likewise present -in thestack gases, these being `formed from iluorine present in lthephosphatic minerals.

This invention contemplates the removal and recovery of the sulphur :andfluorine compounds by iirst scrubbing the stack gases with water, andthen by treatment of -the water and non-soluble gases to recover sulphurand lluorine in desirable forms. The sulphur is recovered Ntre asIhigh-strength sulphuric acid. The fluorine is recovered as a usableuorine product. The invention, therefore, besides alleviating thenuisance of noxious wastes passed into the atmosphere, also provides forthe improvement of the plant economy through recovery and sale ofvaluable by-product materials.

Other objects and advantages of Ithe invention will appear Afrom thefollowing description of preferred embodiments of the process, referencebeing made to the accompanying drawings showing a flow diagram of theprocess in schematic form.

A part of a more or less typical phosphate processing plant is includedin the drawing, there being shown of this plant a rotary kiln 1t) havingthe feed end thereof sealed to a dust chamber 11, the kiln beingrotatably mounted. Kiln 10 is of the usual form, having a steel outershell `and being internally lined with refractory material, such as rebrick. Freshly prepared superphosphate is delivered to the kiln 1@ froma denning unit 12 by a conveyor 13, of any suitable type. A mixer 14,preferably of the pug mill type having a rotatable bladed mixing shaft,mixes -ground phosphate rock (apatite), alkylation grade sulphuric `acid(88% H2804, approximately), phosphoric acid (28% P205), and water toform the superphosphate, which is delivered to the denning unit 12 for ashort period 4to allow the texture of the superphosphate mixture toimprove. In the kiln, the superphosphate is calcined to produce asubstantially lluorine-free, sulphur- `free tricalcium phosphate productsuitable yfor use as a mineral feed supplement for animal feeding. Thecomplete phosphate treating process, here only partially shown, isdescribed in detail in our prior application yfor US. Letters Patent,S.N. 682,801, filed September 9, 1957, and entitled TricalciumPhosphate, which applica- -tion has now been abandoned, acontinuation-impart thereof, S.N. 708,832, filed January 14, 1958, bythe same applicants and having the same title, being now pending.

A stream of combustion and other gases is drawn from the kiln, by a fan16, through dust chamber 11 and connecting duct 17. Fan 16 also draws agas stream from the denning unit 12 through `a duct 1S, duct 18 beingconnected into duct 17 so Ithat the two `gas streams are there mixedtogether. Both of the gas streams, and the mixture, are made up of air,Water vapor, gaseous uorine compounds, sulphur dioxide (SO2), `sulphurtrioxide (S03) and entrained or suspended solid superphosphate iand/ortricalcium phosphate which, in the case 4of the gas stream from thekiln, was not settled out in dust chamber 11. Dust chamber 11 is anenclosed space of relatively large volume, and consequently `small gasvelocity wherein dust particles withdrawn `from the kiln settle out ofthe gas stream.

A duct 20 leads the mixed gas streams from the discharge of -fan 16 to aDustex unit 21. The Dustex -unit is a high-Velocity multiple cycloneseparatory well known in the ar-t, and requires no further description.

From the discharge point of the Dustex unit, the gas stream passesthrough a duct 24 to a heat removal unit (or cooler) 25, from which thegas stream flows through a duct 26 to a rst scrubber 27. A secondscrubber 2S receives the gaseous discharge from scrubber 27 through aduct 29. The process discharge from scrubber 28 is directed through aduct 30 to a dehumidiiier 33, which forms the initial unit of theapparatus for recovering sulphur. i

The plant units and steps which have been described through the Dustexunit 21, are part of the phosphate plant here partially shown. 'Ihepresent process and apparatus has as its iirst unit the heat removalunit 25.

Scrubber 27 has a valid liquid drain conduit 34 leading into a scrubberslurry tank 35. Scrubber 28 has a valved liquid drain conduit 36 leadinginto a liquor recycle tank 37. A pump 40 pumpsslurry from tank 35through valved pipes 41, 42 into the top of scrubber 27. A pump 43 pumpsliquor from tank 37 through valved pipes 44, 45 into the top of scrubber28. A valved crossover pipe 46 connects between pipes 42 and 45 so thatscrubber slurry from tank 37 may be pumped to scrubber 27. A valvedwater supply pipe 48, from a suitable fresh water source, is connectedinto pipe 45. A pump 49 pumps scrubber slurry from tank 35 throughvalved pipes 51, S2 to the tluorine purication portion of the processindicated at 53. A puried iluorine product 54 is delivered from processportion 53.

Fumes from process portion 53 are recycled through `a valved conduit `56to duct 17. Solid non-fluoride phosphate material is returned byconveyor 57 to superphosphate mixer 14. Dust separated from the gasstream by Dustex unit 21 is also returned to mixer 14 by a conveyor 58.

Dehumidifier 33 is the first unit of the sulphur recovery processwherein S02 and S03 are converted to fresh highgrade sulphuric acid. Aduct 60 conveys the scrubbed and dried gases from dehumidiication unit33 to a heat exchanger (or heater) 61, from whence the gases passthrough a duct 62 to a catalytic oxidation chamber 63. A supply of air,the useful component of which for the processis oxygen (02), is passedfrom atmosphere to duct 62 through valved duct 64.

In chamber 63, sulphur dioxide (SO2) is reacted catalytically withoxygen (02) according to the chemical reaction: 2S02-l-02- 2S03. Sulphurtrioxide (S03) entering chamber 63 from duct 62 is unaffected. A usefulcatalytic material for the reaction is comprised of iinely dividedplatinum (Pt) on an aluminum (Al) carrier. The aluminum is supported onporcelain bars or rods a plurality of which are arranged in spacedrelation with chamber 63 so that the contact between the gas stream andcatalyst is extensive. The platinum is a catalyst for-the reactionwhereby sulphur dioxide is converted to sulphur trioxide. Y

The sulphur recovery portion of the plant is of the type conventionallyused for manufacture of sulphuric acid :from sulphur. In theconventional plant, sulphur is lburned in air, forming sulphur dioxideand a small amount of sulphur trioxide, this mixture of gases in airbeing passed over a platinum or vanadium catalyst to convertsubstantially all ofthe sulphur dioxide to sulphur trioxide. Theprincipal difference between the sulphur recovery unit now described andthe conventional unit is that the concentration of sulphur dioxide andsulphur trioxide in the gases of the present unit is more dilute than inthe conventional unit.

After the gases have passed through catalyst chamber 63, the sulphur nowbeing substantially'all in the form of sulphur trioxide, they aredirected by a duct 66 into a through a valved pipe 75 into the top oftower 67, the acid passing down through the tower over the packingtherein to absorb additional sulphur trioxide, and then returningthrough pipe 70 to recycle tank 71.

White or fresh sulphuric acid (H2804) of high strength and purity iswithdrawn from the lower end of tower 67 through a valved pipe 76, andis collected in a suitable storage tank (not shown) from which it isdelivered for use. This acid constitutes the recovered sulphur product,and is usually of a concentration of approximately 97-99% H2804,preferably about 98% H2804, by weight.

Duct 66 has, intermediate of its length, a cooling unit 77, throughwhich the converted gases from chamber 63 pass before they areintroduced into tower 67. As is indicated by the dashed lines 78, 79',the heat removal unit 25, the heat addition unit 61, and the heatremoval unit 77 may be intercombined in a heat exchanging device (notshown) as is known in the art, so that heat required in unit 61 isderived in units 25 and 77. Such heat exchange devices are well known,and no further description thereof should breN required here.

Gases from which the sulphur trioxide had been absorbed in tower 67 passtherefrom through a duct 80 into a fan or blower 81, fan 81 deliveringthe gases through a duct 82 into a stack 83. These gases contain onlytraces of sulphur compounds and present no air pollution hazard, andconsequently can `be safely passed through stack 83 to the atmosphere.No significant waste of sulphur materials is involved in this disposalof the gases.

The sulphur recovery process which has been described, including theunits'designated by reference numerals 33, 60-64, 66-67, 704-83, is `aportion of a conventional sulphuric acid plant :and process which haspreviously been referred to above. This part of the present process hasbeen only described fbrieily since such are well known in the art, therebeing a number of forms available for application to the presentprocess.

The iluorine recovery process is indicated by reference numeral 5'3. Thepreferred form of fluorine recovery for use with the overall recoveryprocess is to recover the iuorine `as sodium fluosilicate, NaZSi-F6. Theinflow to the fluorine recovery process through valved pipes 51,

52 is a solution or slurry which `contains about 12% F, by weight, insolution. The solid material usually suspended in the solution or slurryis gypsum (CaS04-2H20) and/ or -undissolved tu'calcium phosphate(Ca3(P0.,) 2.). The dissolved fluorine is believed present as HF and/orSiF4 and/or HzSiFG. The rst step toward recovering the fluorine is tocontact the solution with silica (Si02) in some form, such as ordinarysaid, in orderto convert the HF also to SiF., and/or H2SiF6. Thechemical reaction sulphur trioxide absorbing tower 67 wherein thesulphur trioxide is absorbed in, or dissolved in, a solution ofrelatively strong sulphur acid.V Tower 67 is Vusually a packed tower ofsome form, being preferably packed with acid-'proof ceramic orV stonepieces. Sulphuric acid solution which has passed downwardly throughtower 67 to absorb sulphur trioxide from the gas stream entering thetower through duct 66 drains out ofthe tower 67 through a valved pipe 70into a sulphuric acid recycle tank 71, the latter containing a supply ofsuch sulphuric acid. Water is added to the acid in tank 71 through avalved pipe 72 which extends from a suitable fresh water supply source.A pump 73 withdraws sulphuric acid from tank 71 Vthrough a valved pipe74, and delivers the acid bines 'withk free water: 3 SiF4-f-2H20-2H2SiF6-i-Si02. Hyrotluosilicic acid (H2SiF6) is precipitated byaddition of soda ash (Na2C03) to the solution after the solution hasbeen treated by, or reacted with, the sand: Na2C03-1- H2SiF6Na2SiF6+H2CO3 (sodium silicofluoride -lcarbonic acid). The carbonic acidreadily decomposes: H2C03 C02 (agas) +H2O. The precipitated product,sodium silicolluori-de (sodium `luosilicate) is removed -from slution byfiltration any may be water-washed to re- -move anycontaminating-materials in the mother liquor from which it is filtered.

The solid impurities in the solution or slurry delivered by pipe 52usually amounts to about 1/2 percent by weight of CaC04.2H20 and/orC213( P04) 2.

The above-described fluoride precipitation method is preferred, sincelluorine is thereby converted to a useful and valuable form which iseasily handled. The abovedescribed uorine precipitation is well known,and a number of other processes'for iluoride separation may be used inplace thereof, these being amply described in the literature. Y

The followingv Table I shows the compositionsV of the materialsat'points of the process described, the/compositions being designated bythe reference numerals designating the various points in the drawings:

be continuously withdrawn from tank 35 through pipes 51, 52 by pump 49to the fluorine purification process 53.

TABLE I Composztzons and Flow Rates at Various Poznls zn the ProcessMaterial compositions (percent by weight) Material ow rates,

liquids, solids and Gases and vapors Point in the process Types ofmaterials Solids slurries, tons (2,000 Cubic feet per P205 CaO H2804 H2OF SO2 S03 in gases 1b.) per day minute or slurries Materials fed tonn'xer Phosphate rock 34. 3 3. 75

Sulfurie acid 90 5 Phosphoric acid 28 0.7 Water 100 Dust via conveyor58.- 38 1.0 Material via conveyor 57. Material in conveyorSuperphosphate 27. 28.8 26. 0 22.0 2. 0 252 1 Material through Gases andsuspended 0.87 5.0 1. 2.0 (Approx. 720 tons/ 35,000 (at 1 atm.,

ducts 17, 18, fan 16, dust. day of gases) 14.4 800 F.). and duet 20.dust.

(F) (6.3)-. (SO2). (36.0) (S03)- (10.8) Material through Gases andsuspended 0.89 5. 1 1. 53 0.2 (Approx. 720 tons/ 32,200 (at 1 atm.,

duct 24. dust. day of gases) 1.4 700 F.).

dust. (0.6) (3.6) (1.1).. Material through (Approx. 140 F.).

duct 30. (0.2)

(36.0) (10.0) Materials through Water dissolved 0.5 2 1.0 12. 0 1.0 50(approx. 140 F.) pipes 51, 52. materials, solids (slurry). (F) (6.0)--(Solids) (0.5) (S04) (P202)- (0.25) Sulphurproduct from H2504 98 68.7(67.2 H2SO4) pipe 7 Fluorine product at 54- NaaSiF 3100 59.5

1 Saturated. 2 As S04. 3 As NaaSiFu.

Referring to Table I in conjunction with the drawing, it is apparentthat the recoveries of sulphur and of iluorine are very high, nearly100%. Using the exemplary production rate of Table I, which is fortricalcium phosphate production at the rate of 156 tons (2000 lbs.) perday out of kiln 10, sulphur is recovered as sulphuric acid at a rate of68.7 tons per day of 98% H2SO4, and iluorine is recovered as sodiumfluosilicate at a rate of 59.5 tons per day. Thus, materials which haveheretofore been wasted as gases or dusts passed to the atmosphere cannow be recovered in useful and pure forms, and in substantial amounts.Furthermore, neither air pollution nor surface water pollution resultsfrom the use of the described process.

With continued reference to Table I and to the drawing, the operation ofScrubbers 27 and 28 is adjusted so that the slurry passing out of tankthrough pipes 51, 52 contains about 12% fluorine by Weight. Scrubbers 27and 28 are preferably each of the type having a liquid stream enteringat the top of a vertically elongate chamber. The gas stream enters thechamber tangentially near the top of the chamber but below the point ofentry of the liquid stream and at a point of restricted flow of theliquid stream. This type of liquid-gas contact results in intimatemixing of the liquid and gas with consequent high scrubbing eiciency. Acommercially available form of this type of scrubber is the Schutle yandKverting scrubber.

The fluorine concentration in the liquid or slurry circulating from tank35, through pipe 41, pump 40, pipe 42, downwardly through scrubbingtower 27, and thence through drain 34 to tank 35, is maintained at about12% F by the introduction of liquid of lower uorine concentrationthrough crossover pipe 46, so that slur-ry of more or less constantfluorine concentration of about 12% F can The uorine concentration inthe liquid or slurry circulating from tank 37 through pipe 44, pump 43,pipe 45, scrubbing tower 28, and through drain 36 to tank 37 is at alower uorine concentration, and is maintained at the lower concentrationby continuous addition of fresh water through pipe 48. Slurry is bledyfrom this circulation stream to the stream circulating through tower 27through crossover pipe 46, as mentioned before.

Thus the circulating liquid or slurry streams through the two towers 27,28 can each be maintained at substantially constant iluorineconcentration, and slurry of substantially constant fluorineconcentration can continuously be withdrawn from tank 35 to fluorinepuriiication process 53. Maintenance of these conditions is easilyregulated by adjustment of the controlling valves and pumps of thesystems.

By proper selection of towers 27, 28, and by proper control of theliquid streams circulating through the towers in the described manner,the eiciency of tower 27 in removal of uorine from the gas streamentering at pipe 26 is in the range of about 85% to about 93%, `andusually the efficiency of towers 27 and 28 together in removal of uorinefrom gas passed through both towers is in the range of about 94% toabout 100%, and usually 96%.

lFurther illustrating the etiiciency of the process, the phosphate rockand phosphoric acid fed to mixer 14 contain (156 3.75/l00) plus (65O.7/100), or 6.3 tons per day of fiuorine (F). Virtually all of theuorine is carried with the gas stream from the kiln to Scrubbers 27, 28.The Scrubbers remove from about 94% to 100% of the fluorine from the gasstream for recovery. The fluorine purification unit is nearly 100%ecient. Therefore, the overall fluorine recovery efficiency is `approxi-94% to nearly 100%.

-The efficiency of the sulphur recovery portion of theV .Y

process is in the neighborhood of 98% recovery of sulphur (SO2 and S03)in the gases entering at duct 30, and the waste sulphur gases at stack83 amount to only traces in the exhausted gas stream.

The sulphur recovery portion of the unit diers from ordinary sulphuricacid plants of this type in that there is no recycling of the gasstream.

yIt should be noted that two fans or blowers 16 vand 81 provide for thegas movements throughout the process equipment, being arranged forcontinuous operation throughout.

Essentially, the process comprises scrubbing the gas stream eluent of aphosphate acidulation and calcination plant with an amount of watersuicient to absorb substantially all of the luorine compounds from thegas stream, but leaving Vsubstantially all of the sulphur compounds intheV gas stream. Thus, as has not been known before, a substantiallycomplete separation of the uorine and sulphur compounds is eiected. Thefluorine is recovered, more or less conventionally, by chemical con-Vversion and precipitation, or other suitable manner. The sulphur isrecovered, more or less conventionally, by catalytic conversion to S03and absorption in strong H2804. The invention alfords, for the Yfirsttime, a process l j and apparatus whereby the valuable sulphur anduorine wastes may be efficiently recovered at relatively lowcostcompared with their recovered values.

yWhile preferred embodiments of the process and -apparatus according tothe invention have been shown and described, many modications thereofmay be made yby a -person vskilled in the art without departing from thespirit of the invention, and it is intended to protect by ,LettersPatent all forms of the invention falling within the scope of thefollowing claims.

We claim:

'1. Process for recovering gaseous uorine materials and gaseousrsulphurmaterials `from waste gas mixtures obtained from acidulation and/ orcalcination of phosph'ate minerals comprising lthe following steps:

( l) scrubbing 4a gas mixture containing gaseous iiuorine Imaterials andgaseous sulphur materials with Water to dissolve said fluorine materialstherefrom and withdrawing the scrubbing water; and

(2) separating lthe dissolved fluorine materials from the Water ofscrubbing step (1) by '(3) `subsequently converting the uorine materialsto a form insoluble in said water; and

(4) iowing the gas mixture which has -been scrubbed in scrubbing step(l) in a non-recirculated continuf l dus stream through a` unit forconverting all sulphur materials therein to sulphur trioxide landabsorbing the sulphur trioxide in Isulphuric acid to produce sulphuricacid.

2. Process'according'to claim l, step (3) of claim l including reactingthe absorbed luorine materials with sand to convert HF and F2 to SiF4and HzSiFG, precipitating the SiF., and H2SiF6 by addition of soda ashto precipitate Na2SiF6, iiltering the Na'zSiF from the water in whichthe fluorine materials were absorbed; step (4) of the process alsoincluding drying the scrubbed gas mixture before ilowing it throughIsaid converting unit.

3. Process according to claimv2, said iiuorine materials in the gasmixture including one or more of the materials,

F2, HF, SiF4, and HZSFS, and ysaid sulphur materials in the gas mixtureincluding one or more of the materials,

- so2 and S03.

4. Process according to claim 3, said gas mixture be ing derived from aphosphate acidulation and calcination plant, said gas mixturescontaining phosphate compounds and gypsum in dust form, lsaid processalso including,

prior to step (l), removing the bulk of the dust from the y r gasmixture` in a separator unit of high efhciency, removing the remainderof the dust fromthe gas mixture by adsorbing it in Water together withthe uorine materials, separating the uorine as described according tostep (3), and recycling the dust from the separator unit and theabsorbed dusttto the phosphate plant.

5. Process accordingto claim 4, including cooling the Vvgasmixture priorto scrubbing step (1) to prevent heat- Afing of the scrubbing water,heating the gas'mixture after said drying thereof to improve saidconversion of sulphur Ymaterials to sulphurvtrioxide, and cooling thegas mixture nbefore said absorption of sulphur troxide therefrom toVfacilitate said absorption.

6. Process yaccording to claim 5, said gas heating and cooling beingperformed Vin acounteri'low manner to be complementary, and said waterbeing recycled to scrub said gas mixture to produce a constant fluorine`concentration therein of about 6 to 15% F for withdrawal to said sandreaction.

References Cited in the iile of this patent UNITED STATES PATENTS

1. PROCESS FOR RECOVERING GASEOUS FLUORINE MATERIALS AND GASEOUS SULPHUR MATERIALS FROM WASTE GAS MIXTURES OBTAINED FROM ACIDULATING AND/OR CALCINATION OF PHOSPHATE MINERALS COMPRISING THE FOLLOWING STEPS: (1) SCRUBBING A GAS MIXTURE CONTAINING GASEOUS FLUORINE MATERIAL AND GASEOUS SULPHUR MATERIALS WITH WATER TO DISSOLVE SAID FLUORINE MATERIALS THEREFROM AND WITHDRAWING THE SCRUBBING WATER; AND (2) SEPARATING THE DISSOLVE FLUORINE MATERIALS FROM THE WATER OF SCRUBBING STEP (1) BY (3) SUBSEQUENTLY CONVERTING THE FLUORINE MATERIALS TO A FORM INSOLUBLE IN SAID WATER; AND (4) FLOWING THE GAS MIXTURE WHICH HAS BEEN SCRUBBED IN SCRUBBING STEP (1) IN A NON-RECIRCULATED CONTINUOUS STREAM THROUGH A UNIT FOR CONVERTING ALL SULPHUR 